“If an explosion happens, there’s a risk at all scales,” said Donald Swanson, a USGS volcanologist. “If you’re near the crater, within half a mile, you could be subject to ballistic blocks weighing as much as 10 or 12 tons.” If you’re within several miles, you could be within range of smaller rocks the size of marbles, he said, and 20 miles downwind “you would see fine ash floating from the sky like snow.”

The ongoing surge in volcanic activity on Hawaii’s Big Island, reports today's Washington Post, soon could cause a massive explosion from a volcano’s summit, scientists and Hawaii officials warned Wednesday. Kilauea, the longest-erupting volcano on the planet, has displaced some 1,700 people and destroyed 36 structures since a shifting flow of underground magma last week burst through the surface in a residential neighborhood about 40 miles from the top of the volcano, unleashing torrents of molten rock and toxic gases.

Scientists with the U.S. Geological Survey’s Hawaiian Volcano Observatory said Wednesday that the movement of magma from the summit out toward the Leilani Estates neighborhood, where 14 fissures have opened in the past week, has caused the lava lake at the top of the volcano to drop in elevation.

When that lava lake meets the groundwater level — which they think is most likely to happen sometime in the next few weeks — it could lead to a series of powerful steam explosions that could shower the surrounding area in 10-ton molten rocks and spew ash as far as 20 miles downwind, the scientists said during a conference call with reporters.

Scientists said the occurrence of an explosion or multiple explosions would be contingent on a rockfall within the volcano’s inner tunnel or conduit — and there have been frequent rockfalls so far in this eruption — once the magma in that tunnel has dropped to meet the groundwater, creating steam.

A rockfall could create a dam, causing the steam to build pressure until it bursts through the dam as an explosion. But the scientists cautioned that they are unable to predict with absolute certainty when, or even if, that would occur.

“We don’t know, and can’t tell precisely, when the conduit will empty of magma below the water table,” Swanson said. “Secondly, we’re uncertain as to how long it will take for conditions to generate an explosion once the conduit has been emptied below the water table. We expect that it’s quite a rapid process, but we really don’t know for certain.”